THE POTENTIAL ROLE OF POLY LACTIC ACID SCAFFOLD AUGMENTED WITH HUMAN BONE MARROW STROMAL CELLS AS A BONE GRAFT EXTENDER IN IMPACTION BONE GRAFTING.

Abstract

Impaction bone grafting with morsellised allograft is a recognized technique to reconstitute loss of bone stock often encountered during revision hip surgery. Concerns over disease transmission, high costs and limited supply has led to interest in synthetic grafts. Poly (lactic acid) (PLA) grafts are attractive to the tissue engineering community as a consequence of their biocompatibility, ease of processing into three-dimensional structures, their established safety as suture materials and the versatility that they offer for producing chemically defined substrates for bone graft matrices. This study set out to examine the potential of PLA scaffolds augmented with human bone marrow stromal cells in impaction bone grafting (IBG).

Methods: In vitro and in vivo studies were performed on impacted morsellised PLA seeded with human bone marrow stromal cells (HBMSC) and compared to PLA alone. In vitro samples were incubated under osteogenic conditions and in vivo samples were implanted subcutaneously into severely compromised immunodeficient mice, both for 4 weeks. In vitro samples were analysed for cell viability, DNA content, specific alkaline phosphatase activity, immunohistochemistry and mechanical shear testing using a cam shear tester. In vivo samples were analysed for cell viability, immunohistochemistry and evidence of neovascularisation and new bone formation using contrast enhance micro computer tomography.

Results: HBMSC survival post impaction, as evidenced by cell tracker green staining, was seen throughout the samples in vitro and in vivo. In vitro there was a significant increase in DNA content (P< 0.001) and specific alkaline phosphatase activity (P< 0.001) in PLA / HBMSC samples compared to impacted PLA alone. Mechanical shear testing of in vitro PLA / HBMSC samples demonstrated a significant increase in shear strength and interparticulate cohesion compared to PLA alone. Immunohistochemistry for type I collagen, osteocalcin, confirmed cell differentiation along the osteogenic lineage in vitro and in vivo. In vivo studies showed a significant increase in blood vessel number and volume penetrating the PLA / HBMSC constructs (32.6 vessels, 1.19mm3, p=0.02, p=0.004) compared to PLA alone (7.6vessels, 0.12mm3). There was a significant relative increase in new bone formation in the PLA / hBMSC constructs (0.47mm3) compared to PLA alone (0mm3), further confirmed with positive staining for osteoid using Goldners Trichrome.

Conclusion: HBMSC seeded onto PLA can withstand the forces of femoral impaction and continue to differentiate and proliferate along the osteogenic lineage. Furthermore PLA / hBMSC constructs in vitro offer a mechanical advantage over PLA alone and in vivo induce neovascularisation and new bone formation. From both a biological and mechanical perspective these studies have demonstrated that PLA is a suitable and beneficial bone graft extender for use in IBG.